   /* 控制下雪 */
   function snowFall(snow) {
       /* 可配置属性 */
       snow = snow || {};
       this.maxFlake = snow.maxFlake || 200;   /* 最多片数 */
       this.flakeSize = snow.flakeSize || 10;  /* 雪花形状 */
       this.fallSpeed = snow.fallSpeed || 1;   /* 坠落速度 */
 }
  /* 兼容写法 */
  requestAnimationFrame = window.requestAnimationFrame ||
      window.mozRequestAnimationFrame ||
      window.webkitRequestAnimationFrame ||
      window.msRequestAnimationFrame ||
      window.oRequestAnimationFrame ||
      function(callback) { setTimeout(callback, 1000 / 60); };

  cancelAnimationFrame = window.cancelAnimationFrame ||
      window.mozCancelAnimationFrame ||
      window.webkitCancelAnimationFrame ||
      window.msCancelAnimationFrame ||
      window.oCancelAnimationFrame;
  /* 开始下雪 */
  snowFall.prototype.start = function(){
      /* 创建画布 */
      snowCanvas.apply(this);
      /* 创建雪花形状 */
      createFlakes.apply(this);
      /* 画雪 */
      drawSnow.apply(this)
  }
  /* 创建画布 */
  function snowCanvas() {
      /* 添加Dom结点 */
      var snowcanvas = document.createElement("canvas");
      snowcanvas.id = "snowfall";
      snowcanvas.width = window.innerWidth;
      snowcanvas.height = window.innerHeight;
      snowcanvas.setAttribute("style", "position:absolute; top: 0; left: 0; z-index: 1; pointer-events: none;");
      document.getElementsByTagName("body")[0].appendChild(snowcanvas);
      this.canvas = snowcanvas;
      this.ctx = snowcanvas.getContext("2d");
      /* 窗口大小改变的处理 */
      window.onresize = function() {
          snowcanvas.width = window.innerWidth;
          snowcanvas.height = window.innerHeight;
      }
  }
  /* 雪运动对象 */
  function flakeMove(canvasWidth, canvasHeight, flakeSize, fallSpeed) {
      this.x = Math.floor(Math.random() * canvasWidth);   /* x坐标 */
      this.y = Math.floor(Math.random() * canvasHeight);  /* y坐标 */
      this.size = Math.random() * flakeSize + 2;          /* 形状 */
      this.maxSize = flakeSize;                           /* 最大形状 */
      this.speed = Math.random() * 1 + fallSpeed;         /* 坠落速度 */
      this.fallSpeed = fallSpeed;                         /* 坠落速度 */
      this.velY = this.speed;                             /* Y方向速度 */
      this.velX = 0;                                      /* X方向速度 */
      this.stepSize = Math.random() / 30;                 /* 步长 */
      this.step = 0                                       /* 步数 */
  }
  flakeMove.prototype.update = function() {
      var x = this.x,
          y = this.y;
      /* 左右摆动(余弦) */
      this.velX *= 0.98;
      if (this.velY <= this.speed) {
          this.velY = this.speed
      }
      this.velX += Math.cos(this.step += .05) * this.stepSize;

      this.y += this.velY;
      this.x += this.velX;
      /* 飞出边界的处理 */
      if (this.x >= canvas.width || this.x <= 0 || this.y >= canvas.height || this.y <= 0) {
          this.reset(canvas.width, canvas.height)
      }
  };
  /* 飞出边界-放置最顶端继续坠落 */
  flakeMove.prototype.reset = function(width, height) {
      this.x = Math.floor(Math.random() * width);
      this.y = 0;
      this.size = Math.random() * this.maxSize + 2;
      this.speed = Math.random() * 1 + this.fallSpeed;
      this.velY = this.speed;
      this.velX = 0;
  };
  // 渲染雪花-随机形状（此处可修改雪花颜色！！！）
  flakeMove.prototype.render = function(ctx) {
      var snowFlake = ctx.createRadialGradient(this.x, this.y, 0, this.x, this.y, this.size);
      snowFlake.addColorStop(0, "rgba(255, 255, 255, 0.9)");  /* 此处是雪花颜色，默认是白色 */
      snowFlake.addColorStop(.5, "rgba(255, 255, 255, 0.5)"); /* 若要改为其他颜色，请自行查 */
      snowFlake.addColorStop(1, "rgba(255, 255, 255, 0)");    /* 找16进制的RGB 颜色代码。 */
      ctx.save();
      ctx.fillStyle = snowFlake;
      ctx.beginPath();
      ctx.arc(this.x, this.y, this.size, 0, Math.PI * 2);
      ctx.fill();
      ctx.restore();
 };
 /* 创建雪花-定义形状 */
 function createFlakes() {
     var maxFlake = this.maxFlake,
         flakes = this.flakes = [],
         canvas = this.canvas;
     for (var i = 0; i < maxFlake; i++) {
         flakes.push(new flakeMove(canvas.width, canvas.height, this.flakeSize, this.fallSpeed))
     }
 }
 /* 画雪 */
 function drawSnow() {
     var maxFlake = this.maxFlake,
         flakes = this.flakes;
     ctx = this.ctx, canvas = this.canvas, that = this;
     /* 清空雪花 */
     ctx.clearRect(0, 0, canvas.width, canvas.height);
     for (var e = 0; e < maxFlake; e++) {
         flakes[e].update();
         flakes[e].render(ctx);
     }
     /*  一帧一帧的画 */
     this.loop = requestAnimationFrame(function() {
         drawSnow.apply(that);
     });
 }
 /* 调用及控制方法 */
 var snow = new snowFall({maxFlake:60});
 snow.start();
